It is always possible to represent an arbitrary radio signal as a sequence of composite (complex) vectors. The real and imaginary parts of the vector sequence correspond to bipolar amplitude modulation (double side band suppressed carrier AM) of a cosinusoidal and sinuosidal carrier wave respectively (quadrature carriers). When wishing to process a radio signal numerically using digital arithmetic implemented in either specific hardware logic or in software on a computer, microprocessor or some other programmable apparatus, it is first necessary to convert the signal in to numerical form with the aid of a A/D-converter (Analogue to Digital converter).
One common method of achieving this is to first resolve the radio signal into its real and imaginary complex vector part, by correlation with locally generated cosine and sine waves in two balanced mixers, and then to digitalize the two results by means of A/D-conversion. Sometimes there is used a variation of this technique, in which the radio signal is sampled pairwise, separated by one quarter period of its centre frequency. This so-called quadrature sampling technique combines the functions of sampling and A/D-conversion with resolution in real and imaginary parts.